Wow. Well I'd just like to thank Garland for pointing out the eyes and throat. If someone is on top of you, and you are lucky enough to have a hand free, isn't the throat a lot closer than the groin? Did you know that the human has no natural reflex to protect the throat? Sure it can be trained in, but it's not there by itself. Men do have natural reflexes to protect the groin though; so if I were ever in this situation (pinned with a hand free, and imminent rape in my future), I would crush the bastard's wind pipe, stomp on his face, and let him suffocate slowly as I broke all his fingers.
Not that I have any strong opinions about rape though.